07 / Concepts
Key Concepts: A Stephen Hawking Glossary
The ideas behind the science, explained in plain language. Event horizon, singularity, entropy, spacetime and more, each defined and connected to Hawking's work.
Hawking's work draws on a handful of recurring ideas, and understanding them makes everything else on this site clearer. This glossary defines each in plain language, with no equations, and links it to where it appears in his discoveries. Use it as a companion to the science, or browse it on its own.
Event Horizon
The boundary around a black hole beyond which nothing, not even light, can escape. A plain-English definition and why it mattered so much to Stephen Hawking.
Read →Singularity
A point where density becomes infinite and the laws of physics break down, found at the centre of a black hole and at the origin of the universe. What it means in plain terms.
Read →Spacetime
The idea, central to Einstein and to Hawking, that space and time are woven into a single four-dimensional fabric that mass and energy can bend.
Read →General Relativity
Einstein's theory of gravity as the curvature of spacetime, the foundation on which almost all of Stephen Hawking's work was built.
Read →Quantum Mechanics
The physics of the very small, where particles behave as probabilities and empty space is never truly empty. The theory Hawking forced to meet gravity.
Read →Entropy & Black Hole Entropy
Entropy is a measure of disorder that never decreases. Its surprising connection to black holes, the Bekenstein-Hawking entropy, was one of Hawking's deepest contributions.
Read →The No-Hair Theorem
The surprising result that a black hole, whatever formed it, can be fully described by just three numbers: its mass, charge and spin.
Read →The Cosmic Microwave Background
The faint afterglow of the Big Bang that fills all of space, and the strongest single piece of evidence that the universe had a hot, dense beginning.
Read →Escape Velocity
The speed needed to break free of a body's gravity. Push it high enough and you arrive at the defining idea of a black hole, where not even light can escape.
Read →Virtual Particles
Pairs of particles that flicker in and out of existence in empty space. This quantum restlessness of the vacuum is the key to how Hawking radiation works.
Read →Trapped Surface
A region of spacetime so curved that even outward-going light is dragged inward. Roger Penrose's key idea, and the engine of the singularity theorems.
Read →The Planck Scale
The almost inconceivably small scale of length and time at which both gravity and quantum mechanics matter at once, and where a theory of quantum gravity is needed.
Read →Redshift
The stretching of light to longer, redder wavelengths. It is how we know the universe is expanding, and a key piece of evidence behind the Big Bang.
Read →Dark Energy
The mysterious something that makes up most of the universe and is driving its expansion to accelerate. One of the deepest open questions in modern cosmology.
Read →Wormholes
Hypothetical tunnels through spacetime connecting distant points, or even universes. A staple of science fiction that also featured in Hawking's serious work.
Read →String Theory & M-Theory
The idea that the fundamental constituents of nature are tiny vibrating strings, and the leading candidate for a theory uniting gravity with quantum mechanics.
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